1. Field of the Invention
This invention relates generally to automatic aperture inspection systems and more particularly to a system and method for automatically inspecting holes in a planar object for defects in size and boundary surfact smoothness.
2. Description of the Prior Art
Metal deposition masks are used by the electronics industry in the manufacture of large scale integrated circuits (LSI). One packaging technique for integrated circuits involves mounting an integrated circuit chip by bonding raised contact pads on a chip to contact pads which have been formed on a matching substrate using a metal deposition mask. These contact pads must be uniform in size and free from contamination to properly interconnect the circuits of the chip to circuits on the substrate. As the complexity of the substrate circuitry and the density of the chips bonded thereto has increased, the cost penalty associated with improper interconnections has also increased. Careful inspection of the deposition masks is therefore necessary to minimize yield loss caused by improperly formed on mispositioned contact pads.
A metal deposition mask is planar in shape with extremely small holes provided therein at positions corresponding to the positions of the contact pads to be formed on one surface of a substrate. To form the contact pads, the substrate and chip wafer with corresponding masks situated thereupon are placed in a metal deposition chamber. The masks allow evaporated metal to be deposited through the holes therein and onto the substrate and wafer only at those positions defined by the holes. The areas of metal thus deposited on the substrate and wafer through the holes in the masks form the contact pads. Since the hole geometry fully defines the contact pads, the uniformity of the resulting contact pads will be a direct result of the uniformity of the holes.
It is therefore necessary that each mask be carefully inspected after manufacture to insure that all holes are properly formed and are free from contamination. The sheer volume of masks that must be inspected dictates that manual inspection is out of the question.
In order to reuse the mask after it is stripped away from the substrate, the metal remaining on the mask is chemically removed in an acid cleaning process. Care must be exercised during the acid cleaning to prevent either incomplete removal of the material which causes completely or partially blocked holes, or over-etching which enlarges the holes. Care must also be exercised in handling the mask to prevent nicking or gouging the holes or contamination. The mask should be inspected after each acid cleaning to insure that the holes have been properly cleaned and are free from residual metal or contamination, and are within size specifications.
Mask inspection is a difficult task. A typical mask may have thousands of holes to be inspected. Inspection by a human operator takes many hours to complete, is subject to fatigue caused errors and is therefore not economically feasible. Additional difficulty arises since defect criteria are subjective and small defects may go unnoticed. When the inspector is under pressure due to volume and yield requirements, it has been shown that there may be considerable divergence between the results he obtains and an unbiased test on the same mask. Even in unbiased tests, many of the smaller defects may be missed and completely blocked holes may be over looked. The common practice of statistical sampling to determine defect density further decreases the confidence lever in the defect density measurement obtained by a human operator.
In addition to inspecting the form, size and position of each hole in the mask, an ideal inspection technique would also inspect the boundary forming edge of each hole for nicks and partial protrusions of metal or contamination. Furthermore, such inspection technique would have objective criteria for the acceptance or rejection of each hole based upon quantifiable and measureable factors, and would perform the inspection rapidly and consistently identifying all known defects for subsequent rework and repair.